1. Field of the Invention
The invention concerns a method for local manipulation of a B1 field in a first region of an examination subject arranged in an examination volume of a magnetic resonance system, and in which, in a calibration measurement, a B1 measurement value, that represents the average B1 field generated by an antenna arrangement of the magnetic resonance system, is integrally determined over at least one specific sub-volume of the examination volume (for example over a defined slice), and in which desired radio-frequency signal parameters (in particular an amplitude of the emitted radio-frequency signals) are predetermined for a subsequent magnetic resonance measurement on the basis of the determined B1 measurement value. The invention also concerns a magnetic resonance system as well as an auxiliary coil element for implementation of such a method.
2. Description of the Prior Art
Magnetic resonance tomography is a wide-spread technique for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, the body or the body part of the patient or test subject to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field that is generated by a basic field magnet of the magnetic resonance system. Rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatial coding during the acquisition of the magnetic resonance images. Moreover, radio-frequency pulses of a defined field strength (known as the “B1 field”) are radiated with radio-frequency antennas into the examination subject. The nuclear spins of the atoms in the examination subject are excited by means of these radio-frequency pulses such that they are deflected from their equilibrium state (parallel to the basic magnetic field) by an amount known as an “excitation flip angle” (also called a “flip angle” for short). The nuclear spins then precess around the direction of the basic magnetic field. The magnetic resonance signals thereby generated are acquired by radio-frequency acquisition antennas. The magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals.
For emission of the required radio-frequency pulses in the patient positioning region, the tomography scanner typically has an antenna structure permanently installed in the housing of the scanner. This radio-frequency antenna is also designated as a “body coil” or “whole-body coil”. It has (for example in the frequently employed “birdcage structure”) a number of conductor rods arranged around the patient space and running parallel to the main field direction that are connected with one another by ferrules (annular conductors) at the facing sides of the coil. As an alternative, there are also other antenna structures permanently installed in the housing such as, for example, saddle coils. Moreover, local coils can also be used that are arranged directly on the body of the patient, but in most cases the local coils are normally used only as acquisition coils. The emission of the radio-frequency pulses for excitation of the spins ensues with the whole-body coil.
An adjustment or calibration measurement, known as a “transmitter adjust measurement”, is typically implemented before the actual magnetic resonance acquisition. In this adjustment measurement the voltage value at the transmission coil that enables a 90° or 180° deflection of the spins is sought in a compensation process. The integral value over a specific excited volume (for example a thicker slice within the examination volume) is typically taken into account in this measurement. The achieved flip angle α is thereby often measured as a B1 measurement value, which is the average flip angle in this volume since the measurement is integrated over the entire volume. This average flip angle can be directly translated into the average B1 field radiated in the sub-volume.
Considerable eddy currents are frequently induced in patients upon radiation of the radio-frequency pulses, in particular given newer magnetic resonance systems with basic magnetic fields that are equal to or greater than three Tesla. As a result, the actual homogeneous radiated B1 field is more or less strongly distorted in the examination volume. In individual cases this can lead to the situation that a reliable magnetic resonance measurement is problematic in specific body regions of the patient and delivers unusable results. A typical problem case is the region of the spinal column of the patient. The B1 field radiated in this region is frequently lower than in the remaining regions of the body. An acquisition of the spinal column image data without field correction would therefore lead to a poor exposure and to a lower signal/noise ratio, i.e. to a poorer contrast.
Local field corrections by the use of, for example, dielectric cushions are conventionally implemented in order to be able influence the structure of the radiated magnetic field in a suitable manner in optimal detail in all regions of the examination volume, in order to thus achieve an optimally good homogeneity of the B1 field in the examination volume. However, the spinal column imaging typically ensues with the use of a local coil arrangement (also called a “spine coil array”) that is placed under the patient lying on his or her back on the examination table. In order to acquire an optimally high signal via this local coil and thus to achieve a good signal/noise ratio, the distance between the local coil and the patient should be optimally small. It is therefore disadvantageous to place dielectric cushions in the region of the spinal column (i.e. between the local coil and the back of the patient) since the distance between coil and patient is thereby increased.